This invention relates to an apparatus and methods for determining particle size distribution of powders, and in particular but not exclusively, to apparatus able to automatically determine the proportion of different particle sizes within a test sample.
Heretofore, many devices of high sensitivity have been developed for particle size measurement. These either rely on direct dimensional measurement and operate by sieving and/or microscopy, or depend on detecting a physical response, i.e. electrozone, various light beam interference techniques or sedimentation. Most of these techniques are either time consuming, require expensive equipment or are capable of analysing small samples only. In addition, the majority are not suitable for online monitoring. This is an important drawback since in the manufacture, for example, of industrial powders as diverse as paint pigments, cement and photocopier toner, there is a requirement to control tightly the particle size distribution during production. An online capability, where size distribution is measured in real time during the manufacturing process, allows immediate readjustment of the process parameters to control the particle size of the material produced.
It is known from GB 2294772 to provide apparatus to address some of these problems by fluidising a powder within a vibrating helical coiled spring and extending this coiled spring so that progressively larger particles are able to leave through the gap between adjacent turns of the coiled spring. A disadvantage of such apparatus is that it cannot readily measure the mass of powder within the coiled spring as this mass is small compared to the total vibrating mass of the coiled spring and ancillary structure.
According to one aspect of the present invention there is disclosed a particle size analysis apparatus comprising: an oscillatory assembly comprising (a) a receptacle for receiving a powder sample entry means for introducing a particle into the receptacle; an opening in the receptacle which may be adjusted in size, adjuster means for adjusting the size of the opening support means allowing the vessel to reciprocate along an arcuate path, bias means to urge the oscillatory assembly towards a datum position; and drive means to induce oscillations of the oscillatory assembly.
It is preferred in such an apparatus that the receptacle is a helical coiled spring defining a lumen for containing the power sample and that most of the mass of the oscillatory assembly is arranged to result in a relatively low moment of inertia of the oscillatory assembly about the axis whilst the coiled spring is mounted at a distance from the axis so that powder within it has a relatively high moment of inertia about the axis. In one embodiment, determination of the change in total moment of inertia of the oscillatory system caused by particles escaping from the coiled spring allows the mass of powder remaining within the coiled spring to be determined.
In an alternative embodiment, mass of the power sample remaining in the receptacle of spring is measured by detecting a rest position of the oscillatory assembly when the drive means is inoperative.
In yet another embodiment, the powder exiting the receptacle may be collected and weighed.
The coiled spring is preferably coil-bound in its unstressed state, so as to be able to retain small particles. However, the overriding requirement is that the gap between adjacent turns of the coiled spring is smaller than the smallest particles of interest. The resolution of the apparatus may be further improved by supporting the coiled spring so as to limit the magnitude of transverse oscillations. Excessively large transverse oscillations cause a periodic additional separation of the coils of the coiled spring which introduces uncertainty as to the effective size of the gap between adjacent turns of the coiled spring. However, some flexing of the coiled spring is desirable to prevent particles from adhering to the coiled spring; the movement produced between adjacent turns by flexing of the coiled spring helps to loosen adhering particles.
The powder to be tested may be introduced into the coiled spring via a funnel inlet. Captive particles such as ballotini spheres may be used within the coiled spring to aid vibro-dispersion of the powder along the coiled spring and to reduce agglomeration of the powder under test within the coiled spring.
A stepper motor may be used advantageously to extend the coiled spring in conjunction with a threaded shaft and a captive nut which is prevented from rotating but is able to move axially of the shaft. One end of the coiled spring may be fixed in relation to the axial direction of the shaft, and the other end fixed relative to the nut, so that rotation of the shaft moves the nut axially of the shaft and thus extends or contracts the coiled spring.
The restoring means is preferably a steel leaf spring although alternatives are possible including a torsion spring, a spring that is coiled or suitable magnetic devices to emulate the action of a spring, i.e. to apply a restoring force proportional to displacement from a datum position. Where a leaf spring is used it is preferably tapered towards the pivot axis.
The pivot axis is preferably horizontal in use and parallel to the longitudinal axis of the spring, although deviations from this orientation are possible. The bearings for the pivot axis are preferably ball bearings, but alternatively may be any suitable low-friction bearing such as xe2x80x9ccup and conexe2x80x9d or other type. Air bearings are also suitable provided transverse float is not excessive, and the use of torsion springs also as bearings allows the integration of the pivotal axis and restoring means. Cup and cone bearings are preferably provided with an axial pre-load by a biasing means by, for example, a coiled compression spring.
The drive means is advantageously provided by an electromagnet or by an electrodynamic actuator. The drive means will typically directly actuate the support although other embodiments are also possible, for example, the drive means could be coupled to the support via a resilient coupling. This latter arrangement would allow a resonator to be driven by a weakly coupled exciter.
In dynamic mass measurement techniques, a detector means detects a characteristic of the oscillating movement, and may be arranged to determine the resonant frequency of the oscillatory movement of either the support structure or the spring. This may be achieved, for example, with an infrared proximity detector or alternatively a capacitive proximity detector or a rotary shaft encoder on the pivotal axis. As an alternative, the detector means could be arranged to detect the amplitude of the oscillations or the phase difference between the drive pulses and the oscillation of the oscillatory assembly.
In a static mass measurement technique, the oscillatory assembly may be allowed to come to a rest position, and the angular orientation of the rest position may be compared to a datum position to derive the mass of the powder sample. Sensitivity may be improved by disconnecting the biasing means and using a resilient element of lower spring constant to oppose the movement of the weight of the powder about the pivot axis.
The particles that leave the spring may be collected into a suitable container. An alternative embodiment would allow particles of different size ranges to be collected into different containers for subsequent analysis, for example, by a microscope and weighing scales.